Dissolved organic matter and sulfide enhance the CH consumption of a psychrophilic lake methanotroph, sp. S3L5C.

Gammaproteobacterial methanotrophic bacteria (gMOB) are dominant methanotrophs in the water column of oxygen-stratified boreal and subarctic lakes and ponds. (Meta)genomic data suggest that, besides methane (CH), gMOB potentially use dissolved organic matter (DOM) and reduced sulfur compounds (e.g., sulfide) as electron sources. To study the DOM and sulfide metabolism of lake gMOB, we subjected a psychrophilic lake water strain, sp. S3L5C, first to different sulfide levels (NaS, 0-5 mM) to test the toxicity, and subsequently, to freshwater DOM (60 mg L) either alone or with sulfide (0.05 mM) at 1 and 20% CH levels. The growth, CH and O consumption, CO production, and mRNA expression patterns of S3L5C were analyzed. Sulfide concentrations of 0-0.5 mM had no effect, while 1 and 5 mM concentrations inhibited the strain's growth. At 20% CH, DOM addition enhanced CH consumption, CO production, and growth of S3L5C, while the addition of sulfide+DOM led to further increases in these variables. The addition of sulfide+DOM enhanced CH consumption even at 1% CH. The effect of DOM on the S3L5C's metabolism was accompanied by enhanced expression of the gene, which has been suggested to mediate the extracellular electron transfer from DOM. Furthermore, the addition of sulfide+DOM enhanced the expression of the and genes encoding dissimilatory sulfide and thiosulfate oxidation, respectively. Together with previous metagenomic data, these results suggest that the usage of DOM and reduced sulfur compounds as electron sources is a trait that enhances methanotrophy among gMOB of boreal and subarctic lakes and ponds.IMPORTANCEGammaproteobacterial methanotrophic bacteria (gMOB) are crucial mitigators of methane emissions of many ecosystems, like boreal and subarctic lakes and ponds. Metagenomic data suggest that besides using methane, gMOB have genetic potential to use dissolved organic matter (DOM) and sulfide, typically present in lakes and ponds, as electron donors. To test the effect of DOM and sulfide on the methane metabolism of gMOB of oxygen-stratified boreal lakes, we subjected our recently isolated lake gMOB strain, sp. S3L5C, to additions of freshwater DOM and sulfide. We show that DOM and sulfide enhance methane consumption and growth of S3L5C. Furthermore, the expression of genes mediating the electron transfer from DOM and sulfide is enhanced. Our results suggest that the usage of DOM and reduced sulfur compounds as electron sources is a trait that enhances methanotrophy among gMOB and adds significantly to the growing body of literature highlighting the enormous metabolic versatility of gMOB.